Generally, when performing data communication with a communication medium being shared among plural terminal stations, each terminal station carrier-senses the communication medium prior to data packet transmission, refrains from transmitting a data packet when it determines that the communication medium is being used (channel busy), and transmits a data packet when it determines that the communication medium is not used (channel idle). This access control method is called CSMA (Carrier Sense Multiple Access). This method is used for a wireless LAN (Local Area Network) or a power line communication system.
Among the plural terminal stations performing data communication, there might be cases where a signal transmitted from one terminal station cannot be directly received by another terminal station, for example, two terminal stations might be too distant from each other to directly receive signals from each other, or an obstacle that blocks the signals might exist between the two terminal stations. The two terminal stations under such situation are called hidden terminals from each other.
Since the above-mentioned carrier sense does not effectively function between the hidden terminals, one terminal station might start data packet transmission while the other terminal station is transmitting a data packet, and in this case, collision of packets occurs in another terminal station located at an intermediate position between the two terminal stations, whereby normal reception cannot be carried out.
In order to solve the above-mentioned problem, there has conventionally been provided a method for ensuring a communication medium by exchanging a transmission request packet (hereinafter referred to as “RTS”)/a transmission path open request packet (hereinafter referred to as “CTS”) between terminal stations performing communication, before data packet transmission (refer to “Wireless LAN Medium Access Control and Physical Layer Specifications, IEEE Std 802.11, August, 1999”: Nonpatent Document 1).
The method of ensuring a communication medium using RTS and CTS is effective as a solution to the problem that occurs between hidden terminals.
However, since the RTS or CTS is originally a command to forbid transmission from a terminal station other than the destination terminal station, incommunicable terminal stations may occur in a chain reaction, depending on the network construction, leading to a reduction in data transmission efficiency.
In order to solve the above-mentioned problem, there is conventionally proposed a communication system in which RTS and CTS are relayed through another terminal prior to data packet communication (refer to Japanese Published Patent Application No. 2002-353975: Patent Document 1). To be specific, when plural radio stations, which are in a network construction shown in FIG. 1, perform communication using RTS/CTS, an RTS including transmission source and destination addresses which is issued from a terminal station is relayed by the respective terminal stations as RTSR (RTS Reeat) before communication, thereby simultaneously establishing plural communications among which no collision of data packets occur, in the communication network.
Further, generally, there is an access control method called TDMA (Time Division Multiple Access) in which plural terminal stations share one frequency alternately for every short period. This method is used for a PDC (Personal Digital Cellular) as a next generation cellular phone system or a PHS (Personal Handy phone System).
In the TDMA method, since a time to be assigned to each terminal station, which is called a slot, and an access timing to the communication medium are controlled by a base station, each terminal station can occupy the communication medium during the slot assigned thereto, whereby throughput is ensured. Accordingly, in contrast to the CSMA method, the TDMA method has no reduction in throughput due to hidden terminals.
However, the TDMA method requires a control station for controlling access time and timing to the communication medium. Further, installation of the control station leads to a complicated system and increased cost, and therefore, it is difficult to use the TDMA method for communication that occurs temporarily.
In order to solve this problem, there is a conventional method in which an access time to the communication medium is divided into access times for plural terminal stations to constitute slots as communication occupation times, and thereafter, each terminal station notifies the neighboring terminal stations of its reception slot according to the amount of reception data, whereby complicated communication band reservation procedure is dispensed with (for example, refer to Japanese Published Patent Application No. 2004-215073: Patent Document 2). Using this method, transmission of sync data can be carried out between arbitrary terminal stations, without providing a base station.
Hereinafter, the communication method disclosed in Patent Document 2 will be described in more detail.
In Patent Document 2, each terminal receives a beacon packet at a specific timing from a neighboring terminal, and synchronization is autonomously achieved between the respective terminals on the basis of the transmission timing of the beacon packet and information required for constituting a slot, which is included in the beacon packet. FIG. 18 is a diagram illustrating the construction of a frame period employed in the conventional communication method. FIG. 19 is a diagram illustrating the positions of reception slots of the respective terminals in the conventional communication method.
With reference to FIG. 18, in the frame period, a beacon slot (S0:BSLT) for transmitting a beacon at a predetermined timing and data slots (S1˜S47:DSLT) for receiving data are arranged, that is, the frame period (FLMP) comprises these 48 pieces of slots. The beacon slot (S0) in the frame period (FLMP) is provided as a position in which each terminal performs transmission of its own beacon signal. The data slots (S1˜S47) in the frame period (FLMP) are provided to clarify that, when each terminal sets at least one reception slot (RSLT), the set reception slot (RSLT) is located in a position corresponding to which timing from the position where the terminal performs transmission of its beacon signal (BCN), in order to prevent each terminal from colliding with the neighboring other terminals.
Each terminal adjusts the beacon packet transmission position and the position of the reception slot (RSLT) so as not to overlap with those of other terminals, and when data reception occurs in the reception slot of the self-terminal, the reception slot is increased or decreased according to the amount of data to be received, thereby constituting the slots without a base station to perform communication of sync data. That is, each terminal previously discloses the reception slot (RSLT) of the self-terminal in the beacon packet, and when performing data communication between terminals, the terminal holding data performs transmission of the data using the reception slot (RSLT) disclosed by the destination terminal to which the data is to be transmitted.
In the communication system disclosed in Patent Document 1, however, since the procedure of relaying RTS and CTS in each terminal station is unclear, it is difficult to use the system for a network whose network construction is dynamically changed. Further, since the communication medium that is ensured by the RTS and CTS is temporarily one, it is impossible to ensure throughput of each terminal station through the whole network.
Further, in the communication method disclosed in Patent Document 2, each terminal station discloses only the reception slot (RSLT) that is the timing at which the terminal station desires to receive data, and performs transmission of data using the reception slot (RSLT) that is disclosed by the destination terminal when performing data communication. Therefore, if there is a hidden terminal in the network, the reception slot (RSLT) might be used by the unspecified terminal, which might cause data collision. More specifically, for example, when performing data transmission from the first terminal to the second terminal, the second terminal starts data transmission using the reception slot (RSLT) disclosed by the first terminal. However, if, at the same time, the third terminal which is a hidden terminal with respect to the second terminal starts data transmission using the same reception slot (RSLT) to the first terminal, collision of the data from the second terminal and the data from the third terminal occurs in the first terminal. That is, the communication method of Patent Document 2 has a problem that reliable and efficient communication cannot be carried out when a hidden terminal exists against communications from plural terminals.
As a method to solve this problem, the above-mentioned RTS/CTS can be applied to the communication method of Patent Document 2. In this case, however, it becomes necessary to repeat the RTS/CTS procedure for every packet, leading to a reduction in throughput.
So, it may be thought that the above-mentioned RTS/CTS procedure is performed not for every packet but at only the beginning of successively transmitted data. Also in this case, however, the band reservation procedure is required eventually, and thereby the feature of Patent Document 2, that is, data communication being carried out by only managing the reception slot of the self-terminal, is lost.